The present invention relates to a zoom lens, and in particular, to a small-sized zoom lens suitable for a lens shutter camera having less restriction of a back focus.
It has been known that a zoom lens having two lens groups composed of a first lens group having positive refracting power and a second lens group having negative refracting power both arranged in this order from an object is right fit for a small-sized zoom lens used in a compact camera, because it is simple in structure and short in total lens length.
However, a zoom lens for a compact camera is requested to have higher variable power, to cover a wider angle and to be more compact, and a conventional zoom lens having two lens groups which has high variable power ratio of 2.5 or more, for example, has not covered a wide angle or has been insufficient in terms of compactness.
An object of the invention is to solve the problems mentioned above and to provide a zoom lens which has a variable power ratio of 2.5 or more and a field angle of 70.degree. or more at a wide angle end and is compact and excellent in image forming power. It is further preferable that the invention provides a compact camera having a zoom lens. In particular, the invention is preferably applied to a zoom lens having two lens groups and to a camera which is equipped with a zoom lens having two lens groups.
Aforementioned objects can be attained by the following structures.
Structure (1): A zoom lens for photographing including a first lens group having positive refracting power and a second lens group having negative refracting power both arranged in this order from an object, conducting variable power by changing a distance between the first lens group and the second lens group, and having a variable power ratio of 2.5 or greater and a field angle of not less than 70.degree. at a wide angle end, wherein the following conditions are satisfied; EQU 1.25.ltoreq.Tw/Tt.ltoreq.1.8 (1) EQU 0.60.ltoreq.fFc/fw.ltoreq.0.9 (2)
wherein,
Tw: Telephoto ratio at a wide angle end PA1 Tt: Telephoto ratio at a telephoto end PA1 fw: Focal length at a wide angle end PA1 fFc: Focal length of a first lens group. PA1 fRc: Focal length of a second lens group PA1 fFc: Focal length of the first lens group. PA1 fRc2: Focal length of a negative meniscus lens PA1 fw: Focal length at a wide angle end
Structure (2): A zoom lens having wherein the conditions in Structure (1) are satisfied and the following conditions are satisfied; EQU 0.65.ltoreq..vertline.fRc.vertline./fFc.ltoreq.1.3 (3)
wherein,
Structure (3): A zoom lens wherein the conditions in Structure (1) are satisfied, the first lens group is composed of a first sub-unit having negative refracting power and a second sub-unit having positive refracting power both arranged in this order from an object across the air distance on the optical axis which is the longest in the first lens group, the first sub-unit is composed of two lenses one of which is a negative lens which is closer to an object, the second sub-unit is composed of one negative lens and one positive lens, or composed of one negative lens and two positive lenses, and the second lens group is composed of two lenses.
Structure (4): A zoom lens wherein the conditions in Structure (1) are satisfied, the first lens group is composed of a first sub-unit having positive refracting power and a second sub-unit having positive refracting power both arranged in this order from an object across the air distance on the optical axis which is the longest in the first lens group, the first sub-unit is composed of two lenses, and the second sub-unit is composed of one negative lens and one positive lens, or of a 3-element type composed of one negative lens and two positive lenses, and the second lens group is composed of two lenses.
Structure (5): A zoom lens wherein the conditions in Structure (1) are satisfied, the second lens group is composed of a negative lens and a negative meniscus lens whose concave surface faces an object both arranged in this order from an object, the aforementioned negative lens is an aspherical lens whose lens thickness on its periphery is thinner than that on a spherical surface having the same paraxial curvature, and the following condition is satisfied; EQU 0.50.ltoreq..vertline.fRc2.vertline./fw.ltoreq.0.9 (4)
wherein,
incidentally, ranges of preferable conditional expressions are as follows. EQU 1.46.ltoreq.Tw/Tt.ltoreq.1.7 (1') EQU 0.72.ltoreq..vertline.fFc.vertline./fw.ltoreq.0.85 (2') EQU 0.75.ltoreq..vertline.fRc.vertline./fFc.ltoreq.0.9 (3') EQU 0.55.ltoreq..vertline.fRc2.vertline./fw.ltoreq.0.7 (4')
Conditional expression (1) stipulates an appropriate range of the ratio of a telephoto ratio at a wide angle end to a telephoto ratio at a telephoto end. The telephoto ratio is defined as a ratio of the total length including a back focus, namely an axial distance from the lens surface closest to an object to the image forming plane to a focal length of a total zoom lens system, and when Lw represents a total length including a back focus at a wide angle end, namely an axial distance from the lens surface closest to an object at a wide angle end to the image forming plane, Lt represents a total length including a back focus at a telephoto end, namely an axial distance from the lens surface closest to an object at a telephoto end to the image forming plane, fw represents a focal length of a total zoom lens system at a wide angle end, and ft represents a focal length of a total zoom lens system at a telephoto end, the following expressions hold, EQU Tw=Lw/fw, Tt=Lt/ft
therefore, the following expression holds. EQU Tw/Tt=(Lw/Lt).multidot.(ft/fw)
For compactness which is one of those to be solved in the invention, it is especially necessary that total length Lt including a back focus at a telephoto end is short.
In addition to this, a great variable power ratio and a short focal length at a wide angle end which represent other problems awaiting solution in the invention are concentrated on the lower limit of the conditional expression (1), and when this lower limit is exceeded, it is difficult to obtain a compact zoom lens having a variable power ratio of 2.5 or more and a field angle of not less than 70.degree. at a wide angle end, especially a compact zoom lens having two lens groups. When an upper limit is exceeded, it is difficult to obtain an excellent image forming power.
Conditional expression (2) stipulates an appropriate range of a focal length of the first lens group.
When a focal length at a wide angle end is short as in the invention, a back focus tends to be short at the wide angle end, and there are easily caused problems that a rear lens system turns out to be big to make a camera big too, or fine dust particles sticking to the rear lens appear as images. When a focal length of the first lens group exceeds the lower limit of the conditional expression (2) to be short, it is difficult to secure a back focus necessary at a wide angle end. Further, a large lateral magnification of the second group is needed, which makes aberration correction difficult. When the upper limit is exceeded, a total length including a back focus is made long at all zooming areas, which makes compactness to be lost.
Conditional expression (3) stipulates an appropriate range of a ratio of a focal length of the second lens group to a focal length of the first lens group. When this range is satisfied, an amount of movement of the second lens group in zooming can be made small to attain further compactness, and fluctuations of astigmatism in zooming can further be restrained, which is preferable.
The lens composition in Structure (3) is advantageous for securing a ratio of brightness of the edge of image field necessary at a wide angle end and a back focus by making the first sub-unit having negative refracting power to precede, and the lens composition described makes it possible to obtain a field angle of about 75 degrees.
The lens composition in Structure (4) is advantageous for making a telephoto ratio to be smaller. In the lens composition of Structure (3) or (4), it is preferable that at least one positive or negative lens in the first sub-unit has an aspherical surface which makes a lens thickness at a peripheral area of the lens to be greater than that relating to the spherical surface having the same paraxial curvature. This aspherical surface makes it possible to correct the distortion at a wide angle end which is excessively positive.
In the lens composition in Structure (5), the second lens group is composed of two negative lenses. There has been known an example wherein the second lens group is composed of one positive lens and one negative lens whose concave surface faces an object. In this case, however, when trying to make the power of the second lens group to be stronger for compactness, the curvature of a concave surface of the negative lens closer to an object is made to be greater, which makes it difficult to polish a glass lens due to its deep surface. This problem can be solved by composing the second lens group with two negative lenses and thereby dispersing the power as in Structure (5). Further, due to lens LRc1 closer to an object having negative power, when there is provided an aspherical surface which makes a lens thickness at a peripheral area of the lens to be smaller than that relating to the spherical surface having the same paraxial curvature, it is possible to increase an amount of displacement of the aspherical surface, while leaving the thin central lens thickness intact. It is therefore possible to correct aberration dynamically while keeping compactness in the case of correction of distortion. Incidentally, when a focal length at a telephoto end is not so long as in the invention, it is possible to limit longitudinal chromatic aberration within a practical range even if the second lens group is composed only of negative lenses. For cost reduction, on the other hand, it is preferable that LRc1 is a plastic lens and LRc2 is a glass lens.
It is further desirable that the first sub-unit of the first lens group has an aspherical lens made of plastic. In addition, it is preferable that a lens closest to an object in the first lens group is a positive lens, and it is more preferable that the lens is an aspherical lens.
Conditional expression (4) stipulates an appropriate range of a focal length of negative meniscus lens LRc2 whose concave surface faces an object in the second lens group in the lens composition of Structure (5). When this range is satisfied, LRc2 can be made to be more appropriate in terms of power, and glass polishing is easier when LRc2 is a glass lens, which is preferable. When the range stated above is satisfied, LRc1 can be made to be more appropriate in terms of negative power, and shift of focus caused by temperature fluctuations can be made less when LRc1 is a plastic lens, which is also preferable.
The lens composition in Structure (6) is a simple one wherein the number of lenses in the second lens group is as small as two lenses, which represents a zoom lens having two lens groups which is suitable for a compact camera.
Incidentally, the invention is especially effective in a zoom lens having two lens groups and a camera equipped with a zoom lens having two lens groups. However, other lens groups may also be incorporated, and a third lens group may be provided on the side of the second lens group closer to an image, for example. It is preferable that the first lens group is provided to be closest to an object. The second lens group is preferably arranged next to the side of the first lens group closer to an image. When providing a stop in a zoom lens, the stop may be provided either between the first lens group and the second lens group, or inside the first lens group.
Incidentally, in the invention, when a plastic lens is used, it is preferable that the plastic lens is made of polycarbonate, acrylic resin, polystyrene or polyolefin.
Though it is desirable that the invention is applied to a camera for a silver halide film use, it is also possible to apply to a digital still camera.